[TableGen] Fix leaking of PhysRegInputs.

Instead of dynamically allocating the vector for PhysRegs, we can
allocate it on the stack and move it into InstructionMemo.

Reviewers: mcrosier, craig.topper, RKSimon, dsanders

Reviewed By: dsanders

Differential Revision: https://reviews.llvm.org/D47461

llvm-svn: 333438